The present invention relates to viscous fluid couplings, and more particularly, to such couplings which are used to drive vehicle radiator cooling fans, wherein the engagement or disengagement of the viscous fluid coupling is controlled in response to a remotely sensed condition, such as coolant temperature.
Viscous fluid couplings have been in commercial use for many years to drive the vehicle radiator cooling fan, especially on vehicles such as automobiles and light trucks. The use of such viscous couplings has been widespread, and quite desirable because such couplings can operate in either an engaged condition, or a disengaged condition, depending upon a sensed temperature condition. Most of the couplings (viscous fan drives) which have been in commercial use include some form of temperature-sensing bimetallic element which senses ambient air temperature adjacent the fan drive, and controls valving within the fan drive in response to the sensed temperature, to achieve either the engaged or the disengaged condition, as is appropriate.
In certain vehicle applications, it has become desirable to sense directly the temperature of the liquid coolant entering the radiator, and to control the viscous fan drive in response to the coolant temperature, whereby the responsiveness of the fan drive is improved, when compared to the conventional fan drive which senses the ambient air temperature.
In order to improve the ability to control such remote-sensing fan drives, it has been considered desirable to convert the sensed temperature into an electrical signal, and to control the engagement of the fan drive by means of that electrical signal. Such electrical control also makes it possible to incorporate more sophisticated logic in the control system. U.S. Pat. No. 4,846,325, assigned to the assignee of the present invention, and incorporated herein by reference, discloses a remote-sensing viscous fan drive in which there is electromagnetic control of the frictional engagement of an armature, relative to a stationary housing, thereby controlling the rotary position of a valve element in the fan drive to cover or uncover a fluid inlet port. In the device of the referenced patent, the electromagnetic control includes a tethered housing through which extends an armature shaft, connected to the valve element of the fan drive. The armature shaft is supported, relative to the stationary housing by means of a pair of bearing sets. There is an additional bearing set disposed between the stationary housing and the cover assembly of the rotating fan drive.
During the development of a commercial product, in accordance with the referenced patent, it was observed that the relatively large speed difference (e.g., 3000 rpm) between the stationary housing and the rotating fan drive could result in excessive wear between the armature shaft and the control housing. In addition, the need for at least three bearing sets substantially increased the complexity and cost of the design, thus making such fan drives economically unacceptable for many vehicle applications.